Three-piece, wound golf balls with balata covers are preferred by most expert golfers. These balls provide a combination of distance, high spin rate, and control that is not available with other types of golf balls. However, balata is easily damaged in normal play, and, thus, lacks the durability required by the average golfer.
In contrast, amateur golfers typically prefer a solid, two-piece ball with an ionomer cover, which provides a combination of distance and durability. Because of the hard ionomer cover, these balls are almost impossible to cut, but also have a very hard "feel", which many golfers find unacceptable, and a lower spin rate, making these balls more difficult to draw or fade. The differences in the spin rate can be attributed to the differences in the composition and construction of both the cover and the core.
Many attempts have been made to produce a golf ball with the control and feel of a wound balata ball and the durability of a solid, two-piece ball, but none have succeeded totally. For example, U.S. Pat. No. 4,274,637 to Molitor discloses two- and three-piece golf balls having covers completely or partially formed from a cellular polymeric material to improve backspin, but does not provide any examples that compare the spin rates of the disclosed golf balls with those of prior art balls.
U.S. Pat. No. 5,002,281 to Nakahara et al. discloses a three-piece solid golf ball having an ionomer cover and a solid core consisting of a soft inner core and a hard outer shell, where the difference in the hardness of the two parts of the core is at least 10 on the JIS-C scale.
Similarly, U.S. Pat. No. 4,781,383 discloses a solid, three-piece golf ball, having an ionomer cover and a core with inner and outer layers, where the inner layer has a diameter of 24 to 29 mm and a Shore D hardness of 15 to 30, and the outer layer has a diameter of 36 to 41 and a Shore D hardness of 55 to 65. The percentage of the ball surface which contacts the club face when the ball is struck is 27 to 35%.
European Patent Application 0 633 043 discloses a solid, three-piece golf ball with an ionomer or balata cover, a center core, and an intermediate layer. The center core has a diameter of at least 29 mm and a specific gravity of less than 1.4. The intermediate layer has a thickness of at least 1 mm, a specific gravity of less than 1.2, and a hardness of at least 85 on the JIS-C scale.
Copending application Ser. No. 08/482,518 now U.S. Pat. No. 5,688,192 employs compressible materials, i.e., gases, in the core of a solid construction golf ball to simulate the effects of trapped air in a wound ball.
None of these disclosures utilizes the unique physical properties of metallocene catalyzed polymers, i.e., polymers produced using single-site metallocene catalysts, which produce polymers with a narrow molecular weight distribution and uniform molecular architecture, so that the order and orientation of the monomers in the polymer, and the amount and type of branching is essentially the same in each polymer chain.
The narrow molecular weight distribution and uniform molecular architecture provides metallocene catalyzed polymers with properties that are not available with conventional polymers, and allow polymers to be produced having unique properties that are specifically tailored to a particular application. The desired molecular weight distribution and the molecular architecture are obtained by the selection of the appropriate metallocene catalyst and polymerization conditions.
Processes for grafting monomers onto polymers and, in particular, polyolefins, are known. European Patent Application No. 0 266 994 of P. C. Wong discloses a process for grafting ethylenically unsaturated monomers, such as unsaturated carboxylic acids and anhydrides and derivatives thereof, onto copolymers of ethylene. The disclosed process comprises the steps of forming an admixture of the copolymer, monomer, and 25 to 3,000 ppm of an organic peroxide having a half-life of about one minute to about 120 minutes at 150.degree. C., and mixing the resultant admixture in an extruder at a temperature above the melting point of the copolymer for a period of time at least four times the half-life of the organic peroxide. The resultant grafted copolymer is then extruded into a shaped article.
U.S. Pat. No. 5,106,916 to Mitchell discloses a process for the catalytic grafting of an ethylenically unsaturated monomer onto a copolymer in which the process of EPA 0 266 994 is modified by the addition of a catalyst comprising water and at least one phosphorous-containing compound selected from the group consisting of compounds of formula HPO(OR.sub.1).sub.2, phosphite compounds of formula P(OR.sub.2).sub.3 and formula (OR.sub.3)P--O--R.sub.4 --O--P(OR.sub.5).sub.2, and disubstituted pentaerythritol diphosphites of formula (R.sub.6 O)P--O.sub.2 --R.sub.PE O.sub.2 --P(OR.sub.7), where O.sub.2 R.sub.PE O.sub.2 is the pentaerythritol moiety, and R.sub.1 -R.sub.7 are specified organic functional groups.
Grafted metallocene catalyzed polymers, which are available commercially, share the advantages of non-grafted metallocene catalyzed polymers, including a narrow molecular weight distribution and uniform molecular architecture. The addition of functional groups to the metallocene catalyzed polymers by grafting allows polymers to be produced having properties that are not available with unfunctionalized metallocene catalyzed polymers or polymers formed without the use of metallocene catalysts.
While different blend combinations of species of one variety of polymer, such as ionomers, have been successfully used in the prior art, different polymers, such as ionomers and balata or other non-ionic polymers have not been successfully blended for use in golf ball covers. In general, prior art blends of polymer components are immiscible, i.e., heterogeneous on a microscopic scale, and incompatible, i.e., heterogeneous on a macroscopic scale, unless strong interactions are present between the polymer components in the mixture, such as those observed between ionomers and polymers containing carboxylic acid groups. In particular, this lack of compatibility exists when an ionomer is blended with a polyolefin homopolymer, copolymer, or terpolymer that does not contain ionic, acidic, basic, or other polar pendant groups, and is not produced with a metallocene catalyst. These mixtures often have poor tensile strength, impact strength, and the like. Hence, the golf balls produced from these incompatible mixtures will have inferior golf ball properties such as poor durability, cut resistance, and so on. In contrast, a compatible blend may be heterogeneous on a microscopic scale, but is homogeneous on a macroscopic scale, and, thus, has useful golf ball properties.
In this regard, U.S. Pat. No. 5,397,840 to Sullivan discloses golf ball covers including a blend of "ionic copolymers" and "non-ionic copolymers". However, the "ionic copolymers" are defined as copolymers of an .alpha.-olefin and a metal salt of an .alpha.,.beta.-unsaturated carboxylic acid, and the "non-ionic copolymers" are copolymers or terpolymers containing ethylene or propylene and acrylic or methacrylic acid monomers. Therefore, strong interactions exist between the metal salts of the "ionic copolymers" and the acrylic or methacrylic acid monomers of the "non-ionic copolymers" that allow compatible blends to be formed. These interactions do not exist in prior art blends of ionomers and polymers that are truly non-ionic or nonpolar, in particular, those polymers produced with a process that does not involve the use of a metallocene catalyst.
U.S. Pat. Nos. 4,986,545; 5,098,105; 5,187,013; 5,330,837; and 5,338,610 to Sullivan disclose golf balls having covers comprising blends of ionomers and modified thermoplastic elastomers, where the thermoplastic elastomers are conventional polymers, i.e., polymers polymerized using catalysts other than metallocene catalysts. The modified polymers include maleic anhydride modified ethylene-propylene copolymers, maleic anhydride modified styrenic block copolymers, maleic anhydride modified ethylene-vinyl acetate copolymers, brominated styrene-isobutylene copolymers, amine modified ethylene-propylene rubber, and polar modified paramethylstyrene-isobutylene copolymers. However, blends of ionomers with modified polyolefins are not exemplified. Although the disclosed balls are said to exhibit enhanced playability, i.e., softness and spin, without sacrificing coefficient of restitution and, thus, carrying distance, all of the exemplified balls have a Riehle Compression in the range of 61 to 43, which corresponds to a PGA Compression range of from 99 to 117. Therefore, even though the disclosed cover materials may be relatively soft, each of the disclosed balls has an extremely high compression, and, thus, would be expected to have a high coefficient of restitution.
As shown in co-pending patent application Ser. No. 08/482,514, metallocene catalyzed polymers and ionomers form compatible blends of useful golf ball properties. However, there is no known prior art disclosure of golf balls incorporating compositions comprising grafted metallocene catalyzed polymers.
Therefore, there is a need in the golf ball art for a golf ball incorporating grafted metallocene catalyzed polymers and blends of grafted metallocene catalyzed polymers and other polymers, such as ionomers, in golf balls. The inclusion of foamed and unfoamed grafted metallocene catalyzed polymers and grafted metallocene catalyzed polymer blends will allow highly durable golf balls to be produced with improved performance and virtually any combination of feel and spin rate.